This invention relates to strain gauging apparatus, and more particularly, to such apparatus having utility in connection with testing or monitoring for purposes of analysis employing advance metallurgical theorems, such as those concerned with propagation of cracks, twins and martensite transformations. In its broader aspects, the invention relates to microphony apparatus for generating electrical signals in response to strain, wave motion, or acoustic signals present at the surface of test specimens or in-service structural members.
Recent advances in metallurgical analysis have created a requirement for strain gauge apparatus, both for purposes of test and operational monitoring, which have higher sensitivity of strain response, faster time-frequency response characteristics, and better spatial resolution than heretofore available with conventional strain gauge apparatus. For most strain gauges, the strain sensitivity is limited to 10.sup.-.sup.5 in./in. Capacitance strain gauges have a sensitivity of 10.sup.-.sup.6 in./in. Most strain gauges are attached to the surface and rapid response is inhibited by the joining material. The smallest conventional strain gauge is the resistance gauge, which is about 1/16 inch in length. The capacitance strain gauge measure strains on an entire specimen.
The approach of using piezoelectric materials in the construction of strain gauge apparatus is known. One known effort has been to cement a thin slab of piezoelectric material to the specimen or structural member, using the bulk properties of piezoelectric materials. This approach involves limitations of sensitivity and time response characteristics, and therefore is not fruitful in significantly advancing strain sensitivity and time response capabilities beyond those of conventional strain gauge apparatuses.
Another known effort using piezoelectric materials has been to use a piezoelectric film deposit in a field effect transistor organization in a way in which the film is used as a semiconductor medium, and straining of the piezoelectric film produces a redistribution of charges along a gate element of the field effect transistor organization. The mode of causing the response by redistribution of charge along the gate element of that approach has limitations in linearity of resonse to strain. In addition, the field effect transistor organization is limited to use in measurements with non-metallic specimens or operational structural members. Further this device is an active device and therefore complicates the problem of instrumentation.